With recent advances in nanoelectronics, products are being developed which apply physical phenomena unique to magnetic materials with minute sizes.
Of these, there have been particularly rapid advances in the field of spin electronics, which utilise the spin of free electrons in magnetic materials.
In the field of spin electronics, spin valve elements utilizing the tunneling magnetoresistance (TMR) effect occurring in a layered structure of a ferromagnetic layer, an insulating layer, and a ferromagnetic layer in order, or utilizing the giant magnetoresistance (GMR) effect occurring in a layered structure of a ferromagnetic layer, nonmagnetic layer (conducting layer), and a ferromagnetic layer in order, are currently regarded as having the greatest possibility of application. FIG. 10 and FIG. 11 show examples of the configuration of spin valve elements of the prior art. Of these, FIG. 10 shows the basic constituent portions of a spin valve element utilizing TMR. This spin valve element has a single insulating layer 24, a pair of a ferromagnetic layer 23 (fixed layer) and a ferromagnetic layer 25 (free layer) sandwiching the insulating layer between them, and electrode layers 21, 27, formed on a substrate 5; to this are further added, as necessary, an antiferromagnetic layer (pinning layer) 22, a capping layer 26, and similar. The magnetization of the fixed layer 23 is fixed by a magnetic coupling with the antiferromagnetic layer 22 and similar. When electrons are passed from the fixed layer 23 to the free layer 25 in this element, a torque acts to cause the spin (the magnetization or magnetic moment) of the free layer 25 to be aligned parallel to the magnetization direction of the fixed layer 23. Conversely, when electrons are passed from the free layer 25 toward the fixed layer 23, a torque acts on the spin of the free layer 25 so as to be antiparallel to the fixed layer 23. Through these actions, the direction of magnetization of the free layer 25 can be controlled by the direction of current. This effect is called spin transfer magnetization reversal. In FIG. 10, an insulating layer 30 is further provided, by means of which the element size is set to a size at which a single magnetic domain is obtained. FIG. 11 shows the basic constituent portions of a spin valve element utilizing GMR. In this case, a difference with an element utilizing TMR in FIG. 10 is that the insulating layer 24 is replaced with a nonmagnetic conducting layer 51; otherwise the functions are basically the same. By utilising these technologies, magnetic random access memory (MRAM) is possible, and so these technologies are attracting the most attention. This MRAM is anticipated as a replacement for conventional DRAM (Dynamic Random Access Memory) and SRAM (Static Random Access Memory).
Further, it is known that when an electric current and an external magnetic field are simultaneously applied to these spin valve elements, microwave oscillation is obtained (see, for example, Y. Suzuki et al, “High-frequency characteristics of spin-transfer elements: oscillation/diode effects and magnetic noise”, Magne, Magn. Soc. Jpn., 2007, Vol. 2 No. 6, p. 282). That is, when for example the current direction is such that a torque acts on the magnetization of the free layer 25 so as to become antiparallel to the magnetization direction of the fixed layer 23, and the external magnetic field is made such that the magnetization of the free layer 25 becomes parallel to the magnetization direction of the fixed layer 23, then under conditions in which the two torques are counterbalanced, high-frequency electromagnetic signals are obtained.
In particular, when the magnetization directions of the fixed layer and the free layer are perpendicular to the film plane, that is, when the magnetocrystalline anisotropy field arising from the crystal anisotropy of the free layer material, the demagnetizing field depending on the film shape, the external magnetic field, and the injected spin are all perpendicular to the plane of the film of the free layer, then the oscillation mechanism is as follows (see id.).
First, in order to describe the state of the physical system in question, the following basic equation, which adds a spin injection term to the Landau-Lifshitz-Gilbert equation, is used. Hereinafter, m and H each represents a vector.dm2/dt=γm2×Heff−αm2×dm2/dt+βST(θ)Im2×(m2×m1)  (14)βST(θ)=g(θ)μ/(Ms×V×e)  (15)
Here m1 and m2 are unit vectors indicating the directions of magnetization in the fixed layer and in the free layer respectively; γ is the gyromagnetic ratio; Heff is the effective magnetic field; α is the Gilbert damping constant; I is the current; θ is the angle made by the vectors m1 and m2; μ is the Bohr magneton; Ms is the saturation magnetization of the free layer; V is the volume of the free layer; and e is the electronic charge. The angle θ (the angle made by m1 and m2) is a polar angle from the z axis direction, when using a polar coordinate representation in which the film plane is taken to be the xy plane and the direction perpendicular to the film plane is the z direction. Also, the symbol “x” appearing between two vectors indicates the outer product of vectors. In equation (15), the third term in the right side expresses the torque of spin injection (spin transfer torque).
By definition, θ=0 indicates that the magnetizations of the fixed layer and free layer are parallel, and θ=π indicates that the two are antiparallel. Also, g(θ) is a function of θ and the spin polarization P, and is represented by the following equations.g(θ)=1/[−4+(P1/2+P−1/2)3(3+cos θ)/4] (for GMR)  (16)g(θ)=P/(1+P2 cos θ) (for TMR)  (17)
From these equations, the entire free layer is approximated by a single spin for a case in which the effective magnetic field of the free layer and the injected spin are both perpendicular to the film plane, and the current and magnetic field oscillation conditions are determined.
Taking the direction of the magnetization in the fixed layer to be the z axis and the film plane to be the xy plane, as in FIG. 12, and expressing equation (14) in polar coordinates, the following are obtained.dm2/dt=Tθeθ+Tφeφ  (18)Tφ=γ(Heff·eθ)  (19)Tθ=−αeffTφ  (20)αeff=α−βST(θ)I(sin(θ)/γ(Heff·eθ))  (21)
Further, Tθ and Tφ, are respectively the polar angle θ and azimuth angle φ components of the torque acting on the vector m2 in addition to the magnetization moment of the free layer. Also, eθ and eφ, are respectively unit vectors in the direction of motion of the radius vector when one of the polar angle θ and the azimuth angle φ is fixed and the other is increased. Also, (·) denotes a inner product of vectors. The effective magnetic field Heff is expressed by the following equation.Heff=Hext+Hu cos θ(22)
Here Hext is the external magnetic field, and Hu is the uniaxial (magnetic) anisotropy field, which is the sum of the magnetocrystalline anisotropy field and the demagnetizing field; both directions are perpendicular to the film plane (the z axis direction in FIG. 12). From this, the following is obtained.Tθ=−αγHuz sin θ cos θ−αγHextz sin θ+βST(θ)I sin(θ)  (23)
Here Huz and Hextz are respectively the z axis components of the vectors Hu and Hext. The conditions for a spin valve element to oscillate are that the torque Tθ in the 8 direction simultaneously satisfy the conditions of the following two equations.Tθ=0  (24)dTθ/dθ<0  (25)
That is, if equation (25) is satisfied in the vicinity of θ=θ0 which satisfies equation (24), θ=θ0 is an energy minimum, and the spin of the free layer then undergoes a stable precessional motion.
As a method of obtaining electromagnetic signals from a magnetization precessional motion, for example, a method is known in which a spin valve element comprising a free layer (first free layer), an insulating layer, and a fixed layer (first fixed layer) is provided with a spin valve element for detection. This spin valve element for detection has, layered together, a ferromagnetic layer (second free layer) in proximity to the first free layer, having an in-plane magnetization which is exchange-coupled with the first free layer; an insulating layer; and, a ferromagnetic layer (second fixed layer), the magnetization of which is fixed in an in-plane direction. By this means, the magnetization in the first free layer which rotates due to the precessional motion causes a rotation of the magnetization in the second free layer, and the magnetoresistance of the spin valve layer for detection arising from the relative angle between this rotating magnetization and the magnetization of the second fixed layer changes periodically, so that an electromagnetic signal equivalent to the period of the precessional motion is obtained.
As explained above, in order to obtain microwave oscillation from these spin valve elements, a DC current and a DC external magnetic field must be simultaneously applied to the spin valve element. Of these, an electromagnet comprising a coil and iron core, a permanent magnet, or similar is necessary to apply the external magnetic field. If ancillary components are included when using such magnets, then components with large volume compared with the spin valve element itself, which normally has dimensions on the order of tens of nanometers, become necessary, and the stronger the external field applied, the larger these must be. Further, electric power is necessary to generate a magnetic field using a current in an electromagnet or similar, and the means for supplying the power also occupies a large volume, power consumption is increased, and larger means are necessary in proportion to the strength of the magnetic field generated. Hence application of a strong magnetic field as the external magnetic field, and use of an external magnetic field itself, are substantial hindrances to efforts to miniaturize the entire element. Thus a driving method of obtaining microwave oscillation from a spin valve element using as weak an external magnetic field as possible, or without using a magnetic field at all, is sought.
Further, there is a problem that the initial state of a spin valve element cannot be identified at the time of element manufacture. For example, when a constant spin transfer current is passed through numerous spin valve elements, elements from which microwave oscillation is obtained and elements from which microwave oscillation is not obtained are intermixed, and there is the problem that characteristics are not constant.
In this way, means have been sought for identifying the initial state of a spin valve element when performing current driving of the spin valve element, and also for stabilising the microwave oscillation characteristics of the spin valve element.
This invention was devised in light of the above circumstances, and has as an object the resolution of at least one among the problems of driving a spin valve element using a weak external magnetic field, or using no external magnetic field at all, to cause microwave oscillation; of obtaining such a spin valve element; and of executing control to enable or disable oscillation of a spin valve element.